The flotation process is used in the separation of particles from mixtures in a finely divided state, suspended in a liquid. For example, in the minerals industry, a suspension of solid particles in water is treated with chemical reagents or collectors which have the effect of making the particles which it is desired to remove, water repellent or hydrophobic, while leaving the remaining particles in a wetted or hydrophilic state. The liquid is fed into a flotation separation cell, which may be in the form of a tank or column, and air is injected in the form of fine bubbles. The hydrophobic particles attach to the air bubbles and rise to the surface of the cell, from which they can be removed by flowing over a lip under the action of gravity, into a launder or channel. The particles which are not collected by the bubbles remain in the suspension and flow out of the bottom of the cell, in the tailings. Frother reagents are often added to the feed liquid in order to assist in the formation of a stable froth on top of the liquid in the cell. Clean water may be applied to the froth layer in order to wash entrained particles downwards into the cell.
Flotation is also used generally for the recovery of fine particles from suspensions in liquids, as in the removal of printing ink from recycled paper; for the removal of particles especially fat and oil droplets from waste waters in the food industry; for removal of particulates in processes for the remediation of contaminated sites; for the treatment of produced water emanating from oil fields; and for the recovery of algae and other organisms from suspensions in fresh water or sea water. For purposes of description, the term ‘air’ may be used to represent the gas, ‘water’ may be used to represent the liquid and the floatable component may be referred to as ‘particles’ or in some cases as the ‘values’. The non-floating component is referred to as ‘gangue’. It is to be understood however that the same principles apply in other systems involving fine particles that are not minerals, dispersed in aqueous or non-aqueous media, being floated with gases other than air.
In earlier technology, flotation has been carried out in mechanical cells in which the liquid is agitated by a rotating impeller and air is introduced in the vicinity of the impeller. The bubble sizes produced in these devices are not necessarily small, being typically in the range 1 to 5 mm in diameter. More recently, flotation has come to be carried out in columns, which have operational advantages in being able to provide better control of the phenomena in the froth. Flotation columns in current use, vary in the aspect ratio. Some are tall relative to their diameter or breadth, with a height-to-diameter ratio of at least 2:1 and up to 10:1 or greater. In these devices the feed slurry is typically injected towards the top of the column, and a stream of bubbles is created by a suitable means such as a sparger, injector, aspirator, nozzle or bubble generator. The objective of these aeration devices is to distribute the bubbles essentially uniformly across the cross-section of the column. Thus as the stream of particle-laden liquid descends down the column, it meets a distributed cloud of small bubbles rising vertically. The individual bubbles collide with and capture the hydrophobic values, and carry them upwards into the froth.
Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field.
In both mechanical cells and columns, the contact between bubbles and particles usually takes place in the liquid in the vessel itself. Thus the reason for the height of tall column cells, is to provide sufficient time for the bubbles to come into contact with particles as they rise in the column. Flotation column cells as described particularly by Finch and Dobby (Column Flotation, Pergamon Press, Oxford, England, 1990), consist of three zones: the froth zone at the very top of the column, typically 1 m in height; the collection zone, where bubble-particle contact occurs, typically 5 to 10 m in height; and the disengagement zone in the base of the column, where the liquid flows out of the column, typically 1 to 2 m in height. Thus the overall height of a column cell is in the range 7 to 13 m. The froth zone must be of sufficient height to allow the gangue particles to drain, and clean wash water is often distributed over the top of the froth or within the froth, to wash the gangue back into the liquid in the flotation cell. The disengagement zone is a quiescent location, where the downward velocity of the liquid is less than the rise velocity of the bubbles which have been introduced higher in the cell, so that the bubbles are able to escape from the exit stream from the column.
Internal bubble generators are known for flotation columns. Some consist of simple distributor pipes with small holes in the walls, or with porous walls. In others, such as the generator of Harach, U.S. Pat. No. 4,911,826, an array of fine nozzles is supported by distributor pipes across the whole cross-section of a tall column. Air and water streams are supplied through headers, and a mixture of air and water is discharged through each fine nozzle. In yet others, air under pressure is supplied to tubes made of an elastic material like rubber. The surface of the elastic tubes is pierced with an array of very fine holes which remain closed when the external pressure is greater than the pressure within the tube. As the internal pressure is increased, the elastic wall stretches and the fine holes enlarge sufficiently to allow the passage of air, which is discharged from the holes in the form of fine air bubbles.
External bubble generators are also known in the tall flotation column cells. Hollingsworth, U.S. Pat. No. 3,371,779, describes a venturi-type aspirator to produce air bubbles into a stream of fresh water which is then introduced into the bottom of a flotation column. Christopherson, U.S. Pat. No. 4,617,113, described how a multitude of venturi aerators can be distributed around a large column. Air is inspired into water flowing through the venturis. In the apparatus of McKay and Foot, U.S. Pat. No. 4,752,383, air and water are premixed at high pressures in a chamber containing beads. The aerated water is then injected into the base of a flotation column through a lance, which has a small orifice at the end. Bacon, U.S. Pat. No. 4,472,271, produced bubbles in slurry taken from the bottom of the flotation cell. The bubbles were made by passing air and slurry through a nozzle. The bubble-laden slurry stream was reintroduced through the wall of the flotation column. Yoon, U.S. Pat. No. 5,397,001, has described a flotation column in which the air is dispersed into slurry in external static mixers. Slurry is taken out of the bottom of the flotation cell and distributed equally among a number of static bubble generators where air is added. The aerated slurry stream is then injected into the flotation column above the external aerators. In the aforementioned devices, the external devices are essentially bubble generators and contact takes place within the column.
Short columns are known, in which the height and diameter are of the same order of magnitude, and the height-diameter ratio in industrial applications may be from 0.2 to 1, to 2 to 1. In these short columns, air is introduced into the feed liquid in an aeration system prior to injection into the column, and it is in this aeration system that contact between bubbles and particles is established. Relatively little contact is effected in the column proper. The aeration system may take the form of a plunging jet, a venturi, a static mixer, or a sparger or porous-walled pipe through which air is introduced in a turbulent fashion into the feed slurry. Examples of such devices are described by Jameson, U.S. Pat. No. 4,938,865; and U.S. Pat. No. 5,332,100; Bahr, Ger. Pat. No. 2,420,482; Imhof, Europ. Pat. No. 1,084,753, and Ludke, U.S. Pat. No. 4,448,681. Because of the high-efficiency contacting in the aeration device, the functions required in the flotation column or tank are much reduced. Thus in principle, there is no need for the collection zone as found in tall column cells, because bubbles and particles have already contacted each other. However, the froth and disengagement zones are required. For present purposes, short flotation column cells of the types described by Jameson and Bahr will be referred to as “intensive” cells. Because there is no need for the collection zone, the intensive cells have significant advantages over the tall column cells, emanating from the much reduced size.
All of the aforementioned inventions describe processes to disperse air bubbles into a liquid which may or not contain suspended particles. However, none of these bubble-generating devices place any form of flow restriction that can be used to control or influence the pressure in the air-liquid mixture after formation. It can be advantageous to control the pressure at which the bubbles are formed, both in absolute terms and also in terms relative to the pressure at which they are to be used in the flotation vessel. For example, when bubbles are generated by the breakup of a supply of air in a shear flow such as exists in the throat of a venturi, or in a static mixer, the size of the resulting bubbles is a function of the local void fraction, which is the ratio of the volume of gas under local pressure conditions, to the total volume of gas and liquid. It is generally desirable to minimize coalescence of bubbles after formation, because it is well known that the rate of capture of particles by bubbles diminishes as the bubble size increases, for a constant air/liquid ratio. Bubble swarms that are created in a gas-liquid mixture of low void fraction, are generally more stable, because the rate of coalescence of bubbles is related to the mean distance between the bubbles, which in turn is related to the void fraction. For the same mass ratio of gas to liquid, the volume ratio varies inversely as the absolute pressure. Thus if it is desired to supply a feed liquid with an equal volume of air at the absolute pressure in the flotation cell, it will be advantageous to create the bubbles at a higher pressure than exists in the cell. For example, if the absolute pressure at which bubbles are generated is twice the absolute pressure in the cell, the volume fraction will be one half that in the cell.
This effect was recognised by Amelunxen (CA Patent Specification 2106925), who described an external contactor, a throttle valve for controlling the process pressure within the contactor and a system for injecting air and liquid into the contactor under pressure.
All of the prior art contactors suffer from disadvantages, which can variously relate to: limitations in the amount of air that can be supplied relative to the amount of liquid flowing through the sparger or aeration device; the necessity for small orifices or tubes which readily corrode or become blocked by the particles present in the feed; the necessity for complex and expensive manufacturing processes to provide parts that can withstand the wear associated by high velocity flows; the difficulty of replacing crucial wearing parts in an operating plant; the need for relatively high concentrations of frother or other expensive surface active agent in order to produce small bubbles; high operating costs associated with excessive driving pressures in the liquid and/or the air streams.
There is a range of particle sizes in the feed suspension for which current flotation technologies are efficient. Thus in an intermediate particle size range, between 40 and 150 microns for minerals (and 75 and 350 microns for coal), conventional flotation cells can achieve high recoveries. However, when the size of the particles is less than or greater than the intermediate range, the flotation recovery tends to decrease as the particles become smaller (or larger). For present purposes, “fine” particles are those whose diameter is smaller than the appropriate intermediate size range, i.e. those between 0 and 40 microns for minerals, and 0 and 75 microns for coal; “ultrafine” particles are those at the lower end of the “fine” range; and “coarse” particles are those whose diameter is greater than 150 microns for minerals, and 350 microns for coal.
The inventor of the present invention has found that improved flotation of fine particles can be achieved by reducing the bubble size, increasing the gas supply rate relative to the flow rate of particles, and increasing the shear intensity or energy dissipation rate in or adjacent the contacting device. The rate of recovery is related to the rate at which the particles collide with the bubbles. Since the inertia of the particles varies inversely as the cube of the diameter, as the particles become smaller, so finer particles tend to follow the fluid streamlines around the bubbles and the probability of attachment is reduced as the size decreases. The recovery of fine particles can be improved by using smaller bubbles and by increasing the rate of shear in the contacting system (N Ahmed and G J Jameson, “The effect of bubble size on the rate of flotation of fine particles”, Int. J. Mineral Processing, 14, (1985), 195-215.). A substantial improvement in the performance of a typical flotation machine can be expected if the bubble size is reduced. Accordingly, it has been recognised by the inventor that for high-efficiency flotation a source of fine bubbles, typically in the range 400 microns in diameter or smaller, be provided, in a high-energy dissipation rate environment.
For coarse particles, the reduction in recovery as the particle size increases is due to the inability of bubbles and hydrophobic particles to stay in contact with each other in a highly-turbulent environment. The bubbles tend to move to the centre of vortices or eddies in the flotation cell and the particles are flung away from the bubbles by centrifugal forces. High recoveries of coarse particles are favoured by a high gas fraction in the slurry suspension, by low levels of turbulence in the region below the froth layer. It is also favourable to provide a means to levitate the coarse particles so that their upwards passage towards the froth is assisted by an upwards motion of liquid in the region beneath the froth.
It is the purpose of the present invention to provide simple, efficient and economic means to overcome the difficulties and inefficiencies in known flotation technologies, by generating fine bubbles and bringing them into contact with the particles to be floated, and controlling the resulting gas-solid-liquid mixture so as to maximise the transfer of hydrophobic particles into the froth and hence into the flotation product.